Cast polymerization methods of polymerizing a plastic lens starting material liquid in a forming mold are widely employed as methods of obtaining plastic lenses by molding plastic into lens shapes. There is one known method of cast polymerization in which two molds (an upper mold and a lower mold) are inserted into an annular gasket and a plastic starting material liquid is cast into the cavity (space) formed by the gasket and the upper and lower molds and polymerized (for example, see Reference 1 (Japanese Unexamined Patent Publication (KOKAI) No. 2008-30431) and Reference 2 (Japanese Unexamined Patent Publication (KOKAI) No. 2009-234111). In east polymerization methods, the shape of the molding surfaces of the molds is transferred to form the shape of the lens surfaces.